Numerous electrochemical processes involve gas-to-liquid or liquid-to-gas transformations. For example, hydrogen-oxygen fuel cells typically utilize the transformation of gaseous oxygen and hydrogen into liquid water at solid-phase, electrically-connected catalysts, like platinum metal.
Many gas-to-liquid or liquid-to-gas processes are most effectively carried out by so-called Gas Diffusion Electrodes (GDEs). At the present time, commercially available GDEs typically comprise fused, porous layers of conductive particles (usually carbon particles) of different size. The outer-most layers typically contain particles of the smallest dimensions, fused together with lesser amounts of hydrophobic PTFE (polytetrafluoroethylene, or Teflon™) binder. The inner-most layers typically contain the largest particles. There may be multiple intermediate layers of intermediary particle size.
The intention of this gradation in particle size within GDEs, from largest in the center to smallest on the outer sides, is to create and control a three-phase solid-liquid-gas boundary within the electrode. This boundary should have the largest possible surface area. The creation of such a boundary is achieved, effectively, by controlling the average pore sizes between the particles, ensuring that the smallest pore sizes are at the edges and the largest are in the center. Since the pores are typically relatively hydrophobic (due to the PTFE binder), the small pore sizes at the edges (e.g. 30 microns pore size) act to hinder and limit the ingress of liquid water into the GDE. That is, water can penetrate only a relatively short distance into the GDE, where the electrochemically active surface area per unit volume, is largest. By contrast, the larger pores in the centre of the GDE (e.g. 150 microns pore size), allow for ready gas transmission at low pressure along the length of the GDE, with the gas then forming a three-way solid-liquid-gas boundary with the liquid water at the edges of the GDE, where the electrochemically active surface area per unit volume is the largest.
Layered porous electrode structures are presently the industry standard for:                (1) conventional free-standing GDEs (for example, of the type used in hydrogen-oxygen PEM fuel cells); and        (2) hybrid GDEs, where a GDE layer has been incorporated within an electrode, typically between a current collector and the gas zone.        
GDEs of this type often display significant technical problems during operation. These largely derive from the difficulty of creating a seamlessly homogeneous particulate bed, with uniform pore sizes and distributions, and uniform hydrophobicity (imparted by the hydrophobic PTFE binder within the GDE). Because of the resulting relative lack of uniformity in the GDE structure, the three-phase solid-liquid-gas boundary created within the GDE may be:                Unstable and fluctuating. The location of the boundary within the GDE may be subject to changing conditions during reaction which cause the boundary to constantly re-distribute itself to new locations within the GDE during operation.        Inhomogeneous. The boundary may be located at widely and unpredictably divergent depths within the GDE as one traverses the length of the GDE.        Inconsistent and ill-defined. At certain points within the GDE, there may be multiple and not a single solid-liquid-gas boundary.        Prone to failure. The boundary may fail at certain points within the GDE during operation, causing a halt to the desired chemical reaction. For example, a common failure mode is that the GDE becomes completely filled with the liquid phase, thereby destroying the three-phase boundary; this is known in the industry as “flooding”. Flooding is a particular problem in fuel cells, such as hydrogen-oxygen fuel cells, that require the feedstock gases to be humidified. Flooding may be caused by water ingress into the gas diffusion electrode via systematic, incremental percolation through the non-homogeneous pores of the electrode, or it may be caused by spontaneous condensation of the water vapour in the feedstock gas stream. In all cases, flooding induces a decline in the voltage output and power generation of such fuel cells.        
Problems of this type are not conducive to optimum or enhanced operations and may result in uneven, low-yielding, incomplete or incorrect reactions, amongst others.
Conventional 3D Particulate Fixed-Bed Electrodes and GDEs
At the present time, 3D particulate fixed bed electrodes and gas diffusion electrodes (GDEs) are conventionally fabricated by mixing carbon black and PTFE powders and then compressing the solid mixture into a bulk, porous electrode.
The pore size of the resulting structure may be very roughly controlled by managing the particle size of the particulates used. However, it is difficult to achieve a uniform pore size throughout the electrode using this approach because particles, especially “sticky” particles like PTFE, often do not flow evenly and distribute themselves uniformly when compressed. A wide range of pore sizes are therefore typically obtained. It is, moreover, generally not possible to create structures with uniformly small pore sizes, such as 0.05 μm-0.5 μm in size.
The hydrophobicity of the structure is typically controlled by managing the relative quantity of PTFE incorporated into the structure. The PTFE holds the structure together and creates the required porosity. However, its quantity must be carefully controlled so as to impart the electrode with an appropriately intermediate hydrophobicity. An intermediate hydrophobicity is needed to ensure partial, but not complete water ingress. In the case of GDEs, this is needed to thereby create a solid-liquid-gas boundary within the carbon black matrix that makes up the electrode.
This method of constructing 3D particulate fixed bed electrodes and gas diffusion electrodes creates some significant practical problems when operating such electrodes in industrial electrochemical cells, particularly in electro-synthetic and electro-energy (e.g. fuel cell) applications. These problems include the formation of three-way solid-liquid-gas boundaries that are: ill-defined, inconsistent, unstable, fluctuating, inhomogeneous, and prone to failures like flooding.
Problems of this type largely arise from the intrinsic lack of control in the fabrication process, which attempts to create all of the inherent properties of the electrode—including porosity, hydrophobicity, and conductivity—in a single step. Moreover, the fabrication method seeks to simultaneously optimise all of these properties within a single structure. This is often not practically possible since the properties are inter-related, meaning that optimising one may degrade another.
Despite these drawbacks, the approach of combining particulate carbon black and PTFE into a compressed or sintered fixed bed remains the standard method of fabricating GDEs for industrial electrochemistry. This approach is used to fabricate, for example, free-standing GDEs of the type used in hydrogen-oxygen PEM fuel cells.
Even where only a GDE component is required within an electrode, the standard method of fabricating that GDE component is to form it as a compressed, porous layer of particulate carbon black and PTFE.
FIG. 1 (prior art) depicts in a schematic form, a conventional 3D particulate fixed bed electrode or a gas diffusion electrode (GDE) 110, as widely used in industry at present.
In a conventional 3D particulate fixed bed electrode or GDE 110, a conductive element (e.g. carbon particles) is typically combined (using compression/sintering) with a non-conductive, hydrophobic element (e.g. polytetrafluoroethylene (PTFE) Teflon™ particles) and catalyst into a single, fixed-bed structure 110. The fixed-bed structure 110 has intermediate hydrophobicity, good but not the best available conductivity, and a pore structure that is non-uniform and poorly defined over a single region 113. When the 3D particulate fixed bed electrode or GDE 110 is then contacted on one side by a liquid electrolyte and on the other side by a gaseous substance, these physical features bring about the formation of an irregularly-distributed three-phase solid-liquid-gas boundary within the body of the electrode 110, below its outer surface 112 and within single region 113, as illustrated in the magnified view presented in FIG. 1. At the three-phase boundary, electrically connected catalyst (solid phase) is in simultaneous contact with the reactants (in either the liquid or the gas phase) and the products (in the other one of the liquid or gas phase). The solid-liquid-gas boundary within the GDE 110 therefore provides a boundary at which electrochemical liquid-to-gas or gas-to-liquid reactions may be facilitated by, for example, the application of a particular electrical voltage. The macroscopic width of the three-phase solid-liquid-gas boundary is comparable or similar in dimension to the width of the conventional GDE. The thickness of the three-phase solid-liquid-gas boundary in a conventional GDE is typically in the range of from 0.4 mm to 0.8 mm in fuel cell GDEs up to, higher thicknesses, such as several millimeters, in industrial electrochemical GDEs.
These problems generally originate in the physical properties of conventional GDEs and typically render the use of GDEs unviable in most common industrial electrochemical processes. We can illustrate this by considering, as a representative example, the phenomena of flooding described above.
Most modern-day, conventional GDEs have exceedingly low “wetting pressures” that are typically less than 0.1 bar. If a 0.1 bar or greater pressure were applied to the electrolyte, the GDE will then flood in part or completely, resulting in electrolyte leaking out of the cell. This is a significant problem in industrial electrochemistry because many cells, for example, employ water electrolyte with a depth greater than 1 meter in the cell. Water, however, experiences a pressure of 0.1 bar at a depth of 1 meter below the surface, meaning that the electrolyte chamber in the cell would leak if a conventional GDE is used as one of the cell's electrodes without additional means applied to balance the trans-GDE pressure differential along the depth of the GDE.
In other industrial electrochemical cells, electrolyte is routinely pumped around the cell. Unless expensive pressure-compensation equipment is installed, such pumping actions would readily generate a pressure of 0.1 bar or more, thereby causing the cell to leak if a GDE were used as one of the electrodes.
Many industrial electrochemical cells furthermore operate most effectively when the liquid electrolyte is pressurised to, for example, several bars of pressure. If a GDE were used as an electrode, then the gases within the GDE would have to be pressurised to within 0.1 bar of the liquid pressure at all times to avoid flooding and consequential leaking. This is generally not technically or economically feasible.
Many industrial electrochemical processes moreover operate optimally only under higher temperatures (e.g. over 80° C.). However, flooding in GDEs may also be caused by a build-up of water vapour in the gas phase, which may then progressively condense in the GDE until the GDE is filled, in part or completely, with water. This is typically facilitated by the relatively ready wettability of conventional gas diffusion electrodes and occurs under conditions of higher temperature and humidity.
The technical problems associated with currently-available GDEs, along with their high cost and other factors, therefore mean that it is generally commercially and technically unviable to use GDEs in many present-day industrial electrochemical gas-to-liquid or liquid-to-gas processes. The effect of this is two-fold:                (1) Potential industrial efficiencies in production are not realised. A key problem with avoiding the use of GDEs is that the technical and other efficiencies associated with their use are not realised. This is true even for processes that should, theoretically, be dramatically improved by the use of GDEs as electrodes. For example, the chlor-alkali process, which is estimated to consume 2% of the world's electricity and is one of the most widely used industrial electro-synthetic processes, does not generally employ GDEs due to at least some of the above discussed problems, even though their use could otherwise dramatically cut energy consumption. Instead, conventional electrodes, with all of their attendant inefficiencies are still routinely employed.        (2) Small-scale, decentralized, “on-site” production is disfavoured despite its potential efficiencies. Many industrial electrochemical gas-to-liquid or liquid-to-gas processes cannot be feasibly carried out in small-scale “on-site” generators at the point where they are needed by an industrial user. Instead, such industrial electrochemical processes are limited to very large-scale installations in centralised facilities, whose products (which are often toxic or hazardous) must then be transported to the point at which they are needed by industrial users. For example, the chlor-alkali process is still mostly carried out in exceedingly large, centralized plants, with the chlorine distributed in cylinders, pipelines or other means to users.        
A key problem in the example case of the chlor-alkali process is that conventional GDEs leak when used in the chlor-alkali process. Numerous patent specifications have described approaches to overcoming this problem. For example, WO 2003035939, WO 2003042430, and, more recently, WO 2013037902, have described fabrication techniques to create Gas Diffusion Electrodes capable of withstanding 0.1 bar pressure and thereby avoiding leaking when the trans-GDE pressure is appropriately managed.
In summary, there exists a need for new or improved electrochemical cells and/or configurations of electrochemical cells. For example, there is a need for a GDE that overcomes or ameliorates at least some of the technical and/or cost difficulties associated with currently available conventional GDEs, and thereby allows for the development of new or improved electrochemical cells and/or configurations of electrochemical cells. For example, which may be small-scale, “on-site” gas-to-liquid or liquid-to-gas electrochemical cells, devices or reactors. In another aspect, there is a need for electrochemical cells that can better or maximally realise the energy and other efficiencies that may be generally conferred by gas diffusion electrodes upon such processes.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.